In a conventional relay driving apparatus, a predetermined voltage is applied to a relay coil to turn on a relay contact and thereafter a voltage is continuously applied to keep the relay contact turned on. The predetermined voltage must be set to be sufficient to turn the relay contact to its ON state from its OFF state. If this same voltage is continuously applied thereafter, the relay coil will overheat due to heat generation in the relay coil. Therefore, it is a general practice to set a first voltage Va applied to the relay coil to turn on the relay contact to be high, and set a second voltage Vb applied thereafter to keep the relay contact turned on to be lower than the first voltage Va.
The relay contact, however, tends to turn off due to vibrations or the like, particularly when the relay is used in a vibrating environment such as a vehicle. JP 63-62052 (JP-A-57-55026) proposes a relay driving apparatus which detects a turn-off of a relay contact in spite of a continued supply of a voltage to a relay coil.
One example of such a relay driving apparatus is shown in FIG. 5. When an ON signal is applied to a relay driving apparatus J1 through an input terminal J1a, a timer circuit J2 produces a high level signal for a predetermined period t1 which is required to fully turn on a relay contact J9 from the OFF state. This high level signal is applied to a Va-voltage instruction circuit J4 through an OR circuit J3. The Va-voltage instruction circuit J4 responsively produces a first voltage Va. This first voltage Va is applied to a relay coil J8 of a relay J7 through an OR circuit J5 and an output circuit J6. Thus, a relay contact J9 is turned on to drive an electric load J12 by magnetic flux generated by the relay coil J8 in response to the first voltage Va.
After the predetermined period t1, the timer circuit J2 changes its high level signal to a low level signal thereby to disable the Va-voltage instruction circuit J4 to continue to produce the first voltage Va. An AND circuit J10 having an inverting input terminal, however, produces a high level signal. A Vb-voltage instruction circuit J11 responsively produces a second voltage Vb. This second voltage Vb is applied to the relay coil J8 through the OR circuit J5 and the output circuit J6. As a result, the relay contact J9 is kept turned on by magnetic flux generated by the relay coil J8 in response to the second voltage Vb.
If the relay contact J9 turns off in its ON state due to vibrations or the like, the voltage at the junction between the relay contact J9 and the load J12 fluctuates. This voltage is applied to a timer circuit J15 through a wire harness J13 and an amplifier J14 having an inverting input terminal. When the voltage on the wire harness J13 falls due to turn-off of the relay contact J9, the timer circuit J15 produces a high level signal of the same period t1 after an elapse of a predetermined time period t2. The Va-voltage instruction circuit J4 receives this high level signal through an AND circuit J16 and the OR circuit J3. As a result, the Va-voltage instruction circuit J4 produces the first voltage Va to energize the relay coil J8 again and restore the ON state of the relay contact J9.
The relay J7 and the load J12 are provided apart from the relay driving apparatus J1. Therefore, the wire harness J13 is required to connect the relay J7 and the load J12 to the relay driving apparatus J1, thus adding costs and complexity. For reducing costs and complexity, the above relay contact turn-off detection is limited to only some of a plurality of electric loads.
Further, if the load J12 is an electric motor or the like, the motor continues to rotate for a certain period even after the turn-off of the relay contact J9, and a voltage is applied to the amplifier J14. Therefore, the turn-off of the relay contact J9 cannot be detected accurately.